The Synechocystis Manganese Exporter Mnx Is Essential for Manganese Homeostasis in Cyanobacteria

Abstract

The essential micronutrient manganese (Mn) functions as redox-active cofactor in active sites of enzymes and, thus, is involved in various physiological reactions. Moreover, in oxygenic photosynthetic organisms, Mn is of special importance, since it is central to the oxygen-evolving complex in photosystem II. Although Mn is an essential micronutrient, increased amounts are detrimental to the organism; thus, only a small window exists for beneficial concentrations. Accordingly, Mn homeostasis must be carefully maintained. In contrast to the well-studied uptake mechanisms in cyanobacteria, it is largely unknown how Mn is distributed to the different compartments inside the cell. We identified a protein with so far unknown function as a hypothetical Mn transporter in the cyanobacterial model strain sp. PCC 6803 and named this protein Mnx for Mn exporter. The knockout mutant Δ showed increased sensitivity toward externally supplied Mn and Mn toxicity symptoms, which could be linked to intracellular Mn accumulation. Mn chase experiments demonstrated that the mutant was not able to release Mn from the internal pool. Microscopic analysis of a Mnx::yellow fluorescent protein fusion showed that the protein resides in the thylakoid membrane. Heterologous expression of suppressed the Mn-sensitive phenotype of the mutant Δ Our results indicate that Mnx functions as a thylakoid Mn transporter and is a key player in maintaining Mn homeostasis in sp. PCC 6803. We propose that Mn export from the cytoplasm into the thylakoid lumen is crucial to prevent toxic cytoplasmic Mn accumulation and to ensure Mn provision to photosystem II.

Manganese Homeostasis in Cyanobacteria

“…The high importance of maintaining the Mn homeostasis with its specific spatial pools in a very narrow beneficial window is indicated by physiological analyses in cyanobacteria but also other bacteria, such as Streptococcus pneumoniae. These studies demonstrate that an only three to five fold increased intracellular Mn concentration results in strong to complete growth inhibition [22][23][24]. Table 1.…”

“…The periplasm also serves as storage for conditionally excess Mn in case the cytoplasmic Mn concentration exceeds its physiological threshold. The thylakoid lumen is suggested to function as a second safe storage for sequestration of surplus Mn [22,23].…”

“…FutABC is an ABC-type transporter like MntCAB but typically uses iron (Fe) as a substrate. It is likely that this transporter accepts also Mn with lower affinity and imports it into the cytoplasm in a piggybacking mode [22,30].…”

“…Only very recently a protein facilitating Mn transport from the cytoplasm into the thylakoid lumen was identified. The protein Mn exporter (Mnx; [22]), also known as photosynthesis affected mutant 71 (PAM71; [23]), belongs to the unknown protein family (UPF) 0016. UPF0016 proteins are highly conserved in oxygenic photoautotrophs, pointing to a transport substrate with a special function in oxygenic photosynthesis.…”

“…Mn as the preferred substrate was also demonstrated for the homologous protein Mnx in Synechocystis. The knockout mutant ∆mnx/pam71 was highly sensitive toward treatment with elevated Mn levels in the medium [22,23]. Further analyses revealed that the mutant accumulates Mn inside the cell, shows reduced Mn export activity, is impaired in PSII activity, and mounts a typical Mn intoxication phenotype with reduced chlorophyll a and PSI accumulation [22,23].…”